At the present time, electronic products are used in almost every aspect of life, and the heart of these electronic products is the integrated circuit. Integrated circuits are used in everything from airplanes and televisions to wristwatches.
Integrated circuits are made in and on silicon wafers by extremely complex systems that require the coordination of hundreds or even thousands of precisely controlled processes to produce a finished semiconductor wafer. Each finished semiconductor wafer has hundreds to tens of thousands of integrated circuits, each wafer worth hundreds or thousands of dollars.
Integrated circuits are made up of hundreds to millions of individual components. One common component is the semiconductor transistor. The most common and important semiconductor technology presently used is silicon-based, and the most preferred silicon-based semiconductor device is a Complementary Metal Oxide Semiconductor (“CMOS”) transistor.
The principal elements of a CMOS transistor generally consist of a silicon substrate having shallow trench oxide isolation regions cordoning off transistor areas. The transistor areas contain polysilicon gates on silicon oxide gates, or gate oxides, over the silicon substrate. The silicon substrate on both sides of the polysilicon gate is slightly doped by ion implantation of boron or phosphorus impurity atoms into the surface of the silicon substrate, thereby becoming conductive. These lightly doped regions of the silicon substrate are referred to as “shallow source/drain junctions”, which are separated by a channel region beneath the polysilicon gate.
A curved silicon oxide or silicon nitride spacer, referred to as a “sidewall spacer”, on the sides of the polysilicon gate allows deposition of additional doping to form more heavily doped regions of the shallow source/drain (“S/D”) junctions, which are called “deep S/D junctions”. The shallow and deep S/D junctions together are collectively referred to as “S/D junctions”.
To complete the transistor, a silicon oxide dielectric layer is deposited to cover the polysilicon gate, the curved spacer, and the silicon substrate. To provide electrical connections for the transistor, openings are etched in the silicon oxide dielectric layer to the polysilicon gate and the S/D junctions. The openings are filled with metal to form electrical contacts. To complete the integrated circuits, the contacts are connected to additional levels of wiring in additional levels of dielectric material to the outside of the dielectric material.
In operation, an input signal to the gate contact to the polysilicon gate controls the flow of electric current from one S/D contact through one S/D junction through the channel to the other S/D junction and to the other S/D contact.
As transistors have decreased in size, it has been found that the electrical resistance between the metal contacts and the silicon substrate or the polysilicon has increased to the level where it degrades the performance of the transistors. To lower the electrical resistance, a transition material is formed between the metal contacts and the silicon substrate or the polysilicon. The best transition materials have been found to be cobalt silicide (CoSi2), nickel silicide (NiSi), platinum silicide (PtSi), and titanium silicide (TiSi2).
The silicides are formed by first applying a thin layer of the transition material (e.g., cobalt, nickel, platinum, or titanium) on the silicon substrate above the S/D junctions and the polysilicon gates. The semiconductor wafer is subjected to one or more annealing steps at temperatures above 800° C. and this causes the transition material to selectively react with the silicon and the polysilicon to form the metal silicide. The process is generally referred to as “siliciding”. Since the shallow trench oxide and the sidewall spacers will not react to form a silicide, the silicides are aligned over the S/D junctions and the polysilicon gates so the process is also referred to as “self-aligned siliciding”, or “saliciding”.
However, existing siliciding and saliciding have not succeeded in solving all the problems related to connecting metal contacts to silicon. For example, an n-type area that is heavily doped to a high concentration of arsenic (As) can result in As segregation into the top surface of the NiSi during silicide formation. This As-rich surface then has a propensity to form an insulating oxide that can cause random open circuits with subsequently formed contacts.
Solutions to these problems have been long sought but prior developments have not taught or suggested any solutions and, thus, solutions to these problems have long eluded those skilled in the art.